Electrical transformer



June 26, 1956 J. TE wlNKEL ELECTRICAI.. TRANSFORMER Filed May l. 1952 GE/VT /NVE/VTOR Jon Te Winkel BY /f/ United States Patent O ELECTRICAL TRANsFoRMER Jan Te Winkel, Eindhoven, Netherlands, assignor to Hartford National Bank and Trust Company, Hartford, Conn., as trustee Application May 1, 1952, Serial No. 285,458

Claims Priority, application Netherlands June 13, 1951 4 Claims. (Cl. 336-130) The present invention concerns electrical transforrners. More particularly, the invention relates to electrical transformers for transmission of signals and comprising a ferromagnetic core which is closed or substantially closed and two windings surrounding the said core. Such transformers are frequently used, for example, as low-frequency transformers. When use is made of core materials suitable for the purpose such, for example, as cubric mixedcrystal material of iron oxides and other metallic oxides (ferrite), the said transformers may be used for highfrequency and for the transmisson of a broad range of frequencies of, for example, 10 to 5,000 kcs./sec., for example, in Cartier-wave telephony apparatus. In this case, the two windings' preferably have the Shape of coaxial cylindrical windings. As a rule, an electrostatic screen is provided between the two windings.

A transformer, more particularly for use at high frequencies (higher than 10,000 cs./sec.) fundamentally constitutes a section of a filter which may be so proportioned that the des'ired transmission characteristic is obtained. However, for this purpose it is necessary for the stray inductance of the transformer to be adjustable with a fairlyhigh accuracy. Hitherto, an adjustment equivalent to the said control was obtained for additional cost and with complications by connecting an additional inductance in series with the transformer. o

The object of the invention is to provide a transformer by which the stray inductance may be suitably Controlled in a manner which may readily be realized structurally with little additional cost. v

According to the invention, a transformer for the transmission of signals and comprising a ferromagnetic core which is closed or substantially closed at two windings embracing the said core, is characterized in that for the purpose of adjusting the stray inductance of the transformer a ferromagnetic control body is slidably arranged between the windings in such manner that an adjustable portion of the control body is provided between the windings.

In one advantageous embodiment, which is very simple structurally and which is particularly suitable for use at high frequency and for the transmission of a very wide band of frequencies, the windings are of a cylindrical Shape and provided in such manner that one winding embraces the other, the control body, which may have the shape of a strip, being slidable between the two cylindrical windings in the axial direction thereof. The cylindrical coil formers on which the windings are provided may readily be made of such rigidity that deformation of the coil formers and hence Variation in the adjustment of the control body cannot substantially occur, while the control strip is suificiently locked between the coil formers against shifting, due to friction. The control strip may be constituted by a simple insulating strip, for example of suitable plastic, having provided on it a fiexible layer of ferromagnetic material, for example ferrite powder with a binder.

This will now be described with reference to the acice companying drawing, given by way of example, in which:

Fig. l is a schematic diagram of a known equivalent circuit of a transformer;

Fig. 2 is a schematic diagram, in side elevation (partly broken away), of a transformer according to the present invention;

Fig. 3 is a schematic theoretical diagram of a portion of the transformer of Fig. 2 and serves to explain the operation of the transformer of Fig. 2; and

Fig. 4 is a schematic diagram, in cross-sectional view taken along the line IV-IV of Fig. 2, and on a larger scale than that of Fig. 2.

In the substitution diagram of Fig. l, L1 and L2 indicate the primary and the secondary stray inductances, respectively, of a transformer. The impedance constituted by the common component L of the primary and secondary inductance is very great at the frequencies under consideration and, since L1 and L2 are comparatively small, may be considered negligible with respect to the parallel impedances constituted by the stray capacities C1 and C2 of the primary and the secondary windings, respectively. The circuit is thus equivalent to a section of a filter (tr-section) which, as is well-known, will transmit a determined desired frequency range when suitably proportioned. However, for this purpose it is desirable that the total stray inductance L1+L2=ns should be adjusted with an accuracy at least within 1%.

'This is possible in a simple manner with the transforiner shown in Fig. 2 The coil has a closed ferromagnetic core, preferably of cubic mixed-crystal material of metallic oxides ('ferrite). Its central core rod 1 (hereinafter 'oriefly referred to as core") is embraced by a cylindrical primary winding 3 and a secondary winding 5, which is also cylindrical and which coaxially surrounds the winding 3, said windings being provided on insulating cc-il formers '7, 9, respectively, for example of suitable plastic. Provided between the two windings is a cylindrical, conductive screen 11, which serves for electrostatic screening of the two windings 3 and 5 and which may be constituted by either a thin metal foil (having an axial slit) or an insulating body having deposited thereon a thin metallic layer. For the sake of clarity, a portion of the two coil formers and windings and of the screen is shown broken away.

Before explaning the operation of the device shown in Fig. 2, reference is made to Fig. 3, which illustrates the form of the secondary stray field, which is that portion' of the field produced by current in the secondary winding which is not embraced by the primary winding. In this sectional view, S and P indicate the secondary and the primary windings respectively and K indicates the core embraced by the said windings (only part of the transformer is shown). Said elements correspond to the elements 5, 3 and 1, respectively, of Fig. 2. The screen 11 is not of essential importance and is thus omitted in Fig. 3.

Pig. 3 shows lines of force of the secondary stray field greatly concentrated in that portion of the field which is located within the winding S. This implies that in this portion of the fieid the magnetic reluctance is comparatively high with respect to that in the remaining portion of the stray field. This magnetic reluctance may be, for example, of the total magnetic reluctance in the magnetic circuit.

If a ferromagnetic body A (shown in dotted line in Fig. 3) is introduced into the range of the greatly concentrated lines of force, the magnetic reluctance is locally decreased considerably. The total reluctance in the magnetic circuit likewise decreases, which implies that the stray field becomes Stronger and the stray impedance which is dependent thereon becomes higher. The extent of the increase is a function of the ratio between the amount of ferromagnetic material introduced into the'f field and the space between the two windings P and S (the filling). It is thus possible to control the stray inductance by moving the body A, which may, for example, be in the form of a strip, to a greater or smaller extent into the coil S. Since this body is introdueed into Vthe range of the maximum magnetic reluctance, a sufficiently wide control range may be obtained with a comparatively small '*filling."

The control body or strip in the embodiment shown in Figs. 2 and 4 is constituted by a flexible strip of insulating material 13 having provided on it a flexible layer of ferromagnetic material 15, preferably ferrite powder with a binder. The stray indnctance is increased by pushing the layer 15 further into the winding 5.

In the case shown, the strip 13, 15 is provided between the primary winding 3 and the screen 11. A rccess 17 is provided in a flange 7' of the coil fornier to enable passage of the strip 13, 15. The frictional resistance to movement of the strip is snfficient to keep the strip in position, so that the desired control is obtained in a very simple manner. if desired, the strip may be fixed in position With the use of lacqner. This present adjustrnent is to be understood to be covered by the expression "slidable- As an alternative, it is possible for the control strip to be provided between the screen 11 and the secondary winding 5. i

A ferrornagnetic layer having a cross-section which increases in the longitudinal direction of the strip may be provided throughout the length of the strip to insure a more fine control. The width is preferably constant and the thickness of the layer increases in the direction of its length. lt is thus readily possible to obtain a control range of 16%. If a very large control range is desired, use may be made of a tubular body embracing the core instead of a strip. However, in this case, the core (rod) is required to be considerably longer than the windings in order that sufiicient space is available for sliding the control body.

The embodiment described comprising cylindrical windings is particularly suitable for the transmission of high frequency in a wide band, the minimum stray indnctance being low. However, as an alternative, the two windings could be shaped, for example, in the form of discs, the ferromagnetic control body then being displaceable in the radial direction between said discs.

It is to be understood that the invention is not limited to the details disclosed but includes all such variations and modifications as fall within the spirit of the invention and the scope of the appended clairns.

What I claim is:

l. A transforrner for the transmission of signals comprising a ferromagnetic core which is substantially closed, two ,winding assemblies surrounding said core, one of said assemblies surroundingtthe other assembly, and a ferromagnetic control body for controlling the stray inductance of said transformer, said body comprising a resilient fiexible insulating strip having a flexible ferromagnetic layer provided thereon over at least part of its length, said body being slidably arranged between said winding assemblies and being arcuately formed to resiliently engage said winding assemblies.

2. transformer, as set forth in claim l, wherein said control body comprises a fiexible insulating strip having provided thereon a fiexible ferromagnetic layer having a cross-section which increases in the longitudinal direction of said strip.

3. A transformer, as set forth in claim 2, wherein the thickness of said ferrornagnetic layer increases in the longitudinal direction of said strip, the width of said layer' being substantially Constant.

4. A transformer for the transmission of signals comprising a substantially closed ferromagnetic core, a primary winding assern'oiy surrounding said core, a secondary winding assembly telescoped over said primary winding and a ferromagnetic control body for controlling the stray indnctance of said transformer, said body comprising a resilient fiexible insulating strip having a flexible ferromagnetic layer provided thereon over at least part of its length, the thickness of said ferromagnetic layer increasing in the longitudinal direction of said strip, the width of said layer being Substantially constant, said body being slidably arranged between said winding assemblies and being arcuately formed to resiliently engage said winding assemblies.

References Cited in the file of this patent UNITED STATES PATENTS l,859,115 Summers May 17, 1932 2,141,890 Weis Dec. 27, 1938 2,370,7l4 Carlson Mar. 6, 1945 2,411,370 Fries Nov. 19, 1946 2,4-83,900 Hardenberg Oct. 4, 1949 FOREIGN PATENTS 119,191 Sweden July 8, 1947 593,062 Great Britain Oct. 7, 1947 

1. A TRANSFORMER FOR THE TRANSMISSION OF SIGNALS COMPRISING A FERROMAGNETIC CORE WHICH IS SUBSTANTIALLY CLOSED, TWO WINDING ASSEMBLIES SURROUNDING SAID CORE, ONE OF SAID ASSEMBLIES SURROUNDING THE OTHER ASSEMBLY, AND A FERROMAGNETIC CONTROL BODY FOR CONTROLLING THE STRAY INDUCTANCE OF SAID TRANSFORMER, SAID BODY COMPRISING A RESILIENT FLEXIBLE INSULATING STRIP HAVING A FLEXIBLE FERROMAGNETIC LAYER PROVIDED THEREON OVER AT LEAST PART OF ITS LENGTH, SAID BODY BEING SLIDABLY ARRANGED BETWEEN SAID WINDING ASSEMBLIES AND BEING ARCUATELY FORMED TO RESILIENTLY ENGAGE SAID WINDING ASSEMBLIES. 